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Reticulated Vitreous Carbon as a Field Emission Cathode

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jpsmith123

Thu Jul 16 2015, 01:05AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Has anyone ever done anything with "reticulated vitreous carbon", i.e., machined it, made electrical contact with it, etc.?

I found a paper which claims that a 0.5 mm disk of the material has produced a "sustained" field emission current of 180 mA:

And McMaster Carr sells "carbon foam" which I believe is the same material:

I'm thinking that this stuff might be useful as a field emission cathode in things like home made Xray tubes and Lenard ray tubes, but how would you secure it (mechanically), and how would you make a solid electrical connection to it?

Ash Small

Thu Jul 16 2015, 10:10AM

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Looks interesting. I can't find a supplier this side of the pond, though.

Conundrum

Sun Jul 19 2015, 04:01PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
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There was something about using carbon nanotubes as an emitter, the idea was that each vertical tube was a separate emitter in its own right.

jpsmith123

Sun Jul 19 2015, 04:27PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Hello Ash,

Having finished a few other projects, I'm starting to again think about building some kind of modest electron accelerator, and one of the issues is cathode design. I was considering using carbon nanotubes from this place:

Then I saw the "reticulated vitreous carbon". But how to use it? I'm wondering if it can be copper electroplated and then soldered, for example. I suppose I'll just have to order some of the material from McMaster and experiment with it.

Ash Small

Sun Jul 19 2015, 06:09PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

The first idea that came to my mind was some form of high current HV triode, using a LARGE anode. Some form of high current HV diode should also be feasible.

I'm assuming 'active pumping' during use, for simplicity and ease of modification.

I imagine a fairly high filament current and voltage may be required.

If you order some, JP, could you order some extra, and post me some? I'll cover the cost.

EDIT: Apparently it's quite easy to nickel plate carbon, so if you plate the ends with nickel, then copper plate the nickel, it should be solderable.

jpsmith123

Sun Jul 19 2015, 08:00PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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One of the papers I saw that got me interested in carbon/graphite cathodes was this one:

In sec. 2.2 the authors describe what they call a "pyrolytic fiber carbon" which is used as a field emission cathode in their accelerator. I'm not sure exactly what the cathode material is, but it seems to have exactly the characteristics I'm looking for.

When I was searching and trying to find out what they were using, I came across the RVC material, and as a cathode it seems to have functional properties similar to cathode described in the paper.

Also I recently sent an email to Xin Nanomaterials asking some questions about the use of their carbon nanotubes in FE cathodes, so depending on what I learn about their CNT products, I will either get some nanotubes from them or I'll get some of the carbon foam from McMaster, or possibly some of each. When I get some of the material I'll let you know and I'll gladly send you some (gratis).

(BTW as far as the ebeam "diode" part of it is concerned I'm leaning toward something like this:

or like this:

, where the diode operates in a pulsed mode with a short cathode-to-anode gap, and the volume to be evacuated is relatively small).

Ash Small wrote ...

The first idea that came to my mind was some form of high current HV triode, using a LARGE anode. Some form of high current HV diode should also be feasible.

I'm assuming 'active pumping' during use, for simplicity and ease of modification.

I imagine a fairly high filament current and voltage may be required.

If you order some, JP, could you order some extra, and post me some? I'll cover the cost.

EDIT: Apparently it's quite easy to nickel plate carbon, so if you plate the ends with nickel, then copper plate the nickel, it should be solderable.

Ash Small

Mon Jul 20 2015, 12:31PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

By 'cold cathode', do I take it that there is no filament current required?

The pulse generators I've been looking at recently have used 'inductive voltage adder' technology, with fast SiC MOSFET's to produce pulses of under 100nS, similar to this one:

And this one

although the SANDIA one is probably a bit big for anything we have in mind.

Another project I'm working on, now and again, involves placing a CW multiplier inside a vacuum chamber to contain/control the high voltages (several hundred kV), but that's a pretty conventional LINAC.

Max power allowed this side of the pond before a license is required for an accelerator is >1MeV, but I believe it's higher your side of the pond.

jpsmith123

Sat Jul 25 2015, 03:25AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

wrote ...

By 'cold cathode', do I take it that there is no filament current required?

Yep, it's pure field emission. Depending on details, emission starts at some threshold value of the electric field (magnitude) at the surface, and the emitted current rises exponentially as the field strength increases. As the field strength continues to increase, the current and emission current density continue to increase and at some point a part of the emitting surface starts to vaporize, producing a plasma, which is a prolific electron source itself. This mode of operation is called "explosive emission".

wrote ...

The pulse generators I've been looking at recently have used 'inductive voltage adder' technology, with fast SiC MOSFET's to produce pulses of under 100nS, similar to this one:

And this one

although the SANDIA one is probably a bit big for anything we have in mind.

I've been trying to come up with practical ideas to get up into the multi-megavolt range. When I think about it, most of my ideas seem to be some variation of an "inductive voltage adder". One of these days I'll start a thread to discuss some of the ideas.

wrote ...

Another project I'm working on, now and again, involves placing a CW multiplier inside a vacuum chamber to contain/control the high voltages (several hundred kV), but that's a pretty conventional LINAC.

Speaking of that, I'm trying to find out if Siemens ever built one of their proposed novel electrostatic accelerators as shown here:

Ash Small

Sat Jul 25 2015, 09:36AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I've seen the Siemens accelerator before, not sure if it's actually been built, though.

Having followed Klugesmith's experiments with his electro-magnets, I've started toying with the idea of 'conventional amateur cyclotrons', but I'm not convinced that the cyclotron format is suitable for some of my ideas.

There is a lot of interesting stuff over on the 'Cyclotrons' forum, though.

jpsmith123

Fri Jul 31 2015, 01:10AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I wonder, how high in voltage is it possible (maybe "practical" is a better word) to go by brute force; e.g., using a dual resonant pulse transformer (DRPT) directly driving an electron beam diode?

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